Compositions comprising polyphenylene ether resins and high impact rubber modified polystyrenes are well known in the art and have achieved great commercial significance as engineering thermoplastics from which many important shaped articles are made.
The compositions are disclosed and claimed, for example, in Katchman and Lee, U.S. Pat. Nos. 4,128,602; 4,128,603, and 4,128,604. Typically, they can comprise 40 parts by weight of poly(2,6-dimethyl-1,4-phenylene ether) and 60 parts by weight of a rubber modified polystyrene. In preferred embodiments, the rubber will comprise a high-cis polymerized butadiene in particulate form at a content of about 8 percent by weight of the rubber modified polystyrene, the particle sizes will be of a maximum mean diameter of about 2 microns, and the gel content will be greater than about 22 percent by weight. While such compositions are eminently suitable for many uses, such as making business machine housings and automotive parts and foamed packaging materials, they have a major drawback and that is a tendency to provide blow molded articles with poor surface characteristics and they cannot be used to make large blow-molded parts because of collapsing parisons during the molding process. Experiments have shown that neither keeping all factors constant except for increasing the rubber particle size above 3 microns nor increasing both rubber content to a range of 18-25 percent and particle size above 4 microns (with a substantial content of plasticizing phosphate flame retardant present) provides compositions capable of blow molding large parts, e.g., of greater than 10 pounds by weight. At the present state of the art it is very desirable to provide large blow molded hollow articles comprised of polyphenylene ether resin/rubber modified polystyrene resin compositions because of their inherently high strength, environmental resistance and excellent weatherability and dimensional stability. Special mention is made of doors for houses which could be made without sacrificing trees.
It has now been discovered that such compositions and blow molded articles can be provided in accordance with this invention by providing a composition which has a high ratio of low shear rate viscosity to high shear rate viscosity at the optimum processing temperature for the particular composition. This parameter, which will be referred to herein and in the appended claims as the "R* value" appears to correlate with blow moldability of the compositions and it has been discovered that R* values of above about 16 provide uniquely suitable blow moldable compositions. None of the prior art compositions known to applicants herein which comprise polyphenylene ether resins and rubber modified polystyrene resins have an R* value approaching 16; the conventional material first above mentioned has an R* value of about 10. When attempts are made to blow mold this into full size house doors, 40 pounds of resin are needed, and the parison occasionally collapses making continuous production problematic. On the other hand, if a rubber modified polystyrene is used having a high percentage of rubber content and particle sizes above about 3 microns and if the R* value of the composition is greater than about 16, full size doors are readily blow molded in accordance with this invention, without blow out, without warping and having good corners, in a drastically reduced cycle time, using as little as 24 pounds of resin. In full production at the rate of 600 to 700 pounds per hour, 20 doors can be produced per hour, a very significant advantage.